Cross current control for power converter systems and integrated magnetic choke assembly

ABSTRACT

A cross current control system for multiple, parallel-coupled power converters includes common mode chokes, local cross current feedback controllers, and local converter controllers. Each common mode choke is coupled to a respective power converter. Each local cross current feedback controller is configured for receiving common mode cross currents from a respective local cross current detector, calculating a resultant cross current, and generating a local feedback control signal. Each local converter controller is configured for using a respective local feedback control signal to drive the respective power converter in accordance with a coordinated switching pattern. An integral choke assembly includes a common mode choke and a differential mode choke with common and differential mode choke cores configured with at least one magnetic flux path being shared by magnetic flux generated by common mode coils and differential mode coils.

BACKGROUND OF INVENTION

Paralleling multiple power converters is a common practice in thetelecom and UPS (uninterruptible power supply) industries to increaseoverall system power capacities and to enhance system reliabilities bybuilding redundancy. Typical examples of such power converters aresingle phase or three phase converters comprising inverters, rectifiersand DC/DC converters. Typically all the parallel power converters aregated synchronously and are tied together through isolation transformersto limit the cross current. Synchronous gating implies that the gatecontrols for the parallel converters are perfectly aligned.

Another way to operate the parallel power converters is throughinterleaved gating. Interleaved gating means that the switching patternsof the parallel converters are uniformly phase shifted, rather thansynchronized. Interleaved gating has several advantages such as havingreduced harmonic filter size, increased system efficiency, greatlyenhanced control bandwidth (and thus improved dynamic performance), andpotentially reduced EMI (electromagnetic interference).

Common mode current that circulates among the paralleled multipleconverters or within paralleled converter systems that does notcontribute to the output to the load is typically referred to as “crosscurrent.” Both synchronous and interleaved gating control embodimentstypically result in undesirable cross current with the cross currentbeing more severe in interleaved embodiments. In ideal conditionssynchronous gating does not lead to cross current, but in actualcircuits using synchronous gating cross current exists due to mismatchedcircuit parameters. One way to reduce the cross current is by using anisolation transformer. In embodiments with isolation transformers, theseisolation transformers account for almost one third of the system cost.

The existing techniques for controlling cross current without using anisolation transformer all suffer from certain inherent disadvantages.For example, using current balancers or inter-phase reactors forcontrolling cross current requires design of an inter-phase reactor.Such design cannot be standardized for arbitrary numbers of convertersin parallel.

Another technique of controlling cross current without using isolationtransformers is through use of “combined-mode” current control bytreating two parallel converters as one converter, selecting the“optimum” switching vector, and adding a current balancer. The“combined-mode approach” is not suitable for more than two converters inparallel because the modulator complication level increases drasticallywhen dealing with more than two parallel converters.

It would therefore be desirable to have an improved cross control systemfor interleaved or synchronous operation of multiple power converters,arranged in parallel, without using isolation transformers.

SUMMARY OF INVENTION

Briefly, in accordance with one embodiment of the present invention, across current control system for multiple, parallel-coupled powerconverters comprises common mode chokes, local cross current detectors,local cross current feedback controllers and local convertercontrollers. Each of the common mode chokes is coupled to a respectivepower converter. Each local cross current detector is configured forobtaining common mode cross currents from a respective output line of arespective power converter. Each of the local cross current feedbackcontrollers is configured for receiving the common mode cross currentsfrom respective local cross current detectors, calculating a resultantcross current, and generating a local feedback control signal. Each ofthe local converter controllers is configured for using a respectivelocal feedback control signal to drive the respective power converter inaccordance with a coordinated switching pattern which may compriseeither an interleaved or a synchronous switching pattern with respect tothe other power converters.

In accordance with another embodiment of the invention, a method ofcontrolling cross-current through multiple, parallel-coupled powerconverters comprises providing common mode chokes, each coupled to arespective power converter; and obtaining common mode cross currentsfrom output lines of the power converters. The method further comprisesfor each respective power converter, calculating a resultant crosscurrent by using the respective common mode cross currents, generating alocal feedback control signal by using the resultant cross current, anddriving the respective power converter by using the respective localfeedback control signal in accordance with a coordinated switchingpattern with respect to the other power converters.

In accordance with another embodiment of the invention, an integralchoke assembly comprises a common mode choke and a differential modechoke. The common mode choke comprises a common mode core wound with atleast two common mode coils and a differential mode choke comprises adifferential mode core wound with at least one differential mode coil.The common and differential mode choke cores are configured so that atleast one magnetic path is shared by magnetic flux generated by commonand differential mode coils.

BRIEF DESCRIPTION OF DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 illustrates a cross current control system according to oneembodiment of the invention;

FIG. 2 illustrates a cross current control system for an individualconverter according to one embodiment of the invention;

FIG. 3 illustrates a DC link common mode choke with a common DC bus;

FIG. 4 illustrates an AC link common mode choke with a common DC bus;

FIG. 5 illustrates a DC link common mode choke with separate DC bus;

FIG. 6 illustrates an AC link common mode choke with separate DC bus;

FIG. 7 illustrates an AC link choke comprising an integrated magneticchoke according to one embodiment of the invention;

FIG. 8 illustrates an integrated magnetic structure showing a closedrectangular and an E core according to one embodiment of the inventionand magnetic flux paths generated by coils shown in FIG. 9;

FIG. 9 illustrates the embodiment of FIG. 8 with three common mode coilswound around the closed rectangular core and a respective differentialmode coil on each leg of the E core;

FIG. 10 illustrates an embodiment of the integrated magnetic structurehaving a top closed rectangular core, a bottom closed rectangular coreand three posts;

FIG. 11 illustrates the embodiment of FIG. 10 with the top closedrectangular core wound with three common mode top coils, the bottomclosed rectangular core wound with three common mode bottom coils andthree posts with a respective differential mode coil on each post, andmagnetic flux paths generated by coils shown in FIG. 12;

FIG. 12 illustrates an embodiment of the integrated magnetic chokecomprising a single phase common mode choke and a single phasedifferential mode choke;

FIG. 13 Illustrates an embodiment of an integrated magnetic structureshowing a closed rectangular core and a U core, and magnetic flux pathsgenerated by coils shown in FIG. 14;

FIG. 14 illustrates an embodiment of FIG. 13 with the top closedrectangular core wound with two common mode coils and a U core woundwith two differential mode coils on each leg;

FIG. 15 illustrates an embodiment of an integrated magnetic structurehaving a top closed rectangular core, a bottom closed rectangular coreand two posts, and magnetic flux paths generated by coils shown in FIG.16; and

FIG. 16 illustrates an embodiment of FIG. 15 with the top and bottomclosed rectangular cores wound with common mode top coils and a commonmode bottom coils respectively, and the two posts with a differentialmode coil on each post.

DETAILED DESCRIPTION

One embodiment of the present invention is a cross current controlsystem 1 comprising a multi-converter system 10 (or multiplemulti-converter systems 10), as illustrated in FIG. 1, to limit thecross current among multiple, parallel, power converters 20, operatingin a coordinated fashion without an isolation transformer to drive theload 500. The cross current control system comprises at least two powerconverters and their respective controls which are shown as individualpower converter systems 22, 222, and 322 for purposes of example andeach comprise a common mode choke 60, local cross current detectors 70,a local cross current feedback controller 80 and a local convertercontroller 90, and one or more converters 20.

The common mode choke is particularly useful for reducing the crosscurrent at the switching frequency level caused by asynchronousswitching patterns (created by interleaved control embodiments orimperfect synchronous control embodiments) applied to each of theparallel power converters. Each of the common mode chokes is coupled toa respective power converter. The local cross current detectors obtaincommon mode cross currents from output lines of respective powerconverters and feed them into a summer 72 which outputs the summedcommon mode current 24 (total cross current through the individual powerconverter). The local cross current feedback controllers receive thecommon mode cross currents from respective local cross current detectors(either directly or through the summer), calculate a resultant crosscurrent, and generate a local feedback control signal 34. Each of thelocal converter controllers uses a respective local feedback controlsignal to drive the cross current of the respective power convertertowards zero in accordance with a coordinated (interleaved orsynchronized) switching pattern with respect to the other powerconverters. The local converter controller can be implemented by using aproportional regulator, an integral regulator, or aproportional-integral regulator for driving a respective cross currentto zero. The bandwidth of local converter controller is limited by theswitching frequency of the respective power converter.

The local cross current feedback controller, as discussed above,nullifies lower than switching frequency cross-current due toimperfectly matched circuit parameters, such as filter parameters, powerswitches voltage drop, or gate driver dead-time.

In one embodiment, the cross current control system further comprisesmodulators 100, each of which receives a local converter controllersignal from a respective local converter controller and generates afiring signal for driving a respective power converter. The modulatortranslates a continuous signal from the local converter controller intoa switching signal for driving the power converter.

In a more specific embodiment, the cross current control system of FIG.1 further comprises a global feedforward controller 50, which detectsswitching signals (patterns) 28 of the power converters and generatescounter balance zero-sequence global feedforward control signals 30.Each of the local converter controllers further uses a respective globalfeedforward control signal 30 (in addition to local feedback controlsignal 34) to drive the respective power converter. Global feedforwardcontroller 50 takes the information from all the parallel converters ina single converter system and derives a global feedforward controlsignal for each individual power converter.

In another more specific embodiment, which can be used in conjunction orseparately from the global feedforward controller embodiment, the crosscurrent control system of FIG. 1 further comprises a global feedbackcontroller 40, which receives the common mode cross currents from eachof the power converters (and the summed total cross current 26 acrossmultiple power converters), calculates a resulting global cross current,and generates global feedback control signals 32. Each of the localconverter controllers further uses a respective global feedback controlsignal to drive the respective power converter (in addition to localfeedback control signal 34 and optionally in addition to globalfeedforward control signal 30).

The global feedforward controller is designed to eliminate the lowerfrequency cross currents flowing within one multi-converter system 10,while the global feedback controller is used to control the crosscurrent flowing out from one multi-converter system 10 to othermulti-converter systems 10 (i.e. referring to FIG. 1, cross currentthrough power converter systems 10). In an embodiment having a pluralityof multi-converter systems 10 requiring fast cross current control, allthe three controllers (local cross current feedback controller, globalfeedforward controller and global feedback controller) are particularlyuseful.

FIG. 2 illustrates a specific embodiment of the invention for anindividual power converter system 22. Individual power converter system22 comprises a common mode choke 60 and local cross current feedbackcontroller 80 for controlling the cross current. The functions of otherelements such as local cross current detectors 70, summer 72, localconverter controller 90, and modulator 100 are same as described abovewith respect to FIG. 1.

The cross current control system discussed above applies both tosingle-phase and three-phase multiple power converters. The parallelconverters may be rectifiers, inverters, or DC/DC converters or theircombinations for UPS (uninterruptible power supply) or any other powerconditioning systems.

FIGS. 3-6 illustrate various embodiments of a power converter system.Although the embodiments shown are for double conversion (AC to DC andDC to AC), they are equally applicable to other converter topologies.Inductor 38 reduces the current ripples, and capacitor 36 smoothes DClink voltage generated during the switching operation of the powerconverters. Common mode chokes are illustrated as DC link choke 110 inFIGS. 3 and 5 and AC link choke 120 in FIGS. 4 and 6. In the embodimentof the present invention as shown in FIG. 3 and FIG. 4, the powerconverters share a common DC bus 130. In another embodiment of thepresent invention as shown in FIG. 5 and FIG. 6, the power converterscomprise separate DC busses 130.

In another embodiment of the present invention, the AC link chokecomprises a discrete magnetic choke 120 as shown in FIG. 4 and FIG. 6.

In another embodiment of the present invention the AC link chokecomprises an integrated magnetic choke 140 as shown in FIG. 7. Theintegrated magnetic choke comprises an integrating magnetic structure tocouple a three phase common mode choke 150 and a three phasedifferential mode choke 160. The integrating magnetic structurecomprises a common mode core and a differential mode core. Therespective phases of the three phase common mode choke and three phasedifferential mode choke are connected in series. The integrated magneticstructure minimizes the size and cost of magnetic materials. In onespecific embodiment, for example, material expense is minimized byhaving the common mode core comprise a higher permeability material thanthe differential core.

In one integrated choke embodiment, as shown in FIG. 8 and FIG. 9, thecommon mode core comprises a closed rectangular core 170 wound withthree common mode coils 142, one for each phase, and the differentialmode core comprises an E core 180 wound with a respective differentialmode coil 144 on each leg. The E core has a magnetic flux path 148 asshown in FIG. 8. The legs of the E core face the closed rectangular coreand share a part of magnetic flux path 145 of the closed rectangularcore. The common and differential mode cores are typically held togetherin spaced apart relation by non-magnetic clamps or adhesive (not shown),for example.

In another embodiment of the integrated magnetic structure as shown inFIG. 10 and FIG. 11, the common mode core comprises a top closedrectangular core 190 wound with three common mode top coils 142 and abottom closed rectangular core 200 wound with three common mode bottomcoils 143. The differential mode core comprises three posts 210 as shownin FIG. 11, with a respective differential mode coil 144 on each post asshown in FIG. 12. The three posts are arranged between the top andbottom closed rectangular cores and have a magnetic flux path 178. Theposts share a part of top and bottom rectangular magnetic flux paths 146and 147. The integrated magnetic structure of FIG. 11 results in highercommon mode inductance than the integrated magnetic structure of FIG. 9.

In accordance with another embodiment of the invention, which isparticularly useful in single phase choke embodiments and which isdescribed below for several specific examples, an integral chokeassembly comprises a common mode choke and a differential mode choke.The common mode choke comprises a common mode core wound with at leasttwo common mode coils and a differential mode choke comprises adifferential mode core wound with at least one differential mode coil.The common and differential mode choke cores are configured so that atleast one magnetic flux path is shared by magnetic flux generated bycommon and differential mode coils.

An another embodiment of the integrated magnetic choke for single phaseor DC/DC converters as shown in FIG. 12, comprises an integratedmagnetic structure coupling a single phase common mode choke 162 and asingle phase differential mode choke 164. The single phase common modechoke and single phase differential mode chokes are connected in series.In one embodiment, the integrated magnetic structure comprises a commonmode core and a differential mode core with the common mode corecomprising a higher permeability material than the differential modecore.

In one example, as shown in FIG. 13 and FIG. 14, the common mode corecomprises a closed rectangular core 166 wound with two common mode coils142 and the differential mode core comprises a U core 168 wound with twodifferential mode coils 144 on each leg. The legs of U core face theclosed rectangular core and have a magnetic flux path 169. The legsshare a part of magnetic flux path 145 of the closed rectangular core.

In another example, as shown in FIG. 15 and FIG. 16, the common modecore comprises a top closed rectangular core 172 wound with common modetop coils 142 and a bottom closed rectangular core 174, also wound withcommon mode bottom coils 143. The differential mode core comprises twoposts 176 with a differential mode coil 144 on each post. The two postsare arranged between the top and bottom closed rectangular cores andhave a magnetic flux path 178. The posts share a part of the magneticflux paths 146 and 147 of the top and bottom closed rectangular cores.

The various embodiments of integrated chokes 220 discussed above areuseful in combination with cross current control systems as discussedabove and can be useful in other embodiments as well. For example,integrated choke embodiments are useful for EMI filtering in DC/DCconverters.

While only certain features of the invention have been illustrated anddescribed herein, many modifications and changes will occur to thoseskilled in the art. It is, therefore,to be understood that the appendedclaims are intended to cover all such modifications and changes as fallwithin the true spirit of the invention.

What is claimed is:
 1. A cross current control system for multiple,parallel-coupled power converters, the cross current control systemcomprising: common mode chokes, each coupled to a respective powerconverter; local cross current detectors, each configured for obtainingcommon mode cross currents from a respective output line of a respectivepower converter; local cross current feedback controllers, eachconfigured for receiving the common mode cross currents from respectivelocal cross current detectors, calculating a resultant cross current,and generating a local feedback control signal; and local convertercontrollers, each configured for using a respective local feedbackcontrol signal to drive the respective power converter in accordancewith a coordinated switching pattern with respect to the other powerconverters.
 2. The system of claim 1, further comprising a globalfeedforward controller configured for detecting switching patterns ofthe power converters and generating counter balance zero-sequence globalfeedforward control signals, wherein each of the local convertercontrollers is configured for using a respective global feedforwardcontrol signal to drive the respective power converter.
 3. The system ofclaim 2, further comprising a global feedback controller configured forreceiving the common mode cross currents from the local cross currentdetectors, calculating a resulting global cross current, and generatingglobal feedback control signals, wherein each of the local convertercontrollers is configured for using a respective global feedback controlsignal to drive the respective power converter.
 4. The system of claim1, further comprising modulators, each configured for receiving a localconverter controller signal from a respective local converter controllerand generating a firing signal for driving a respective power converter.5. The system of claim 1, further comprising a global feedbackcontroller configured for receiving the common mode cross currents fromthe local cross current detectors, calculating a resulting global crosscurrent, and generating global feedback control signals, wherein each ofthe local converter controllers is configured for using a respectiveglobal feedback control signal to drive the respective power converter.6. The system of claim 1, wherein the power converters are configured toshare a common DC bus.
 7. The system of claim 1, wherein the powerconverters comprise separate DC busses.
 8. The system of claim 1 whereinthe common mode chokes comprise DC link chokes.
 9. The system of claim 1wherein the common mode chokes comprise AC link chokes.
 10. The systemof claim 9, wherein the AC link chokes comprise discrete magneticchokes.
 11. The system of claim 9, wherein the AC link chokes compriseintegrated magnetic chokes.
 12. The system of claim 11, wherein eachintegrated magnetic choke comprises an integrating magnetic structurecoupling a three phase common mode choke and a three phase differentialmode choke.
 13. The system of claim 12, wherein each integratingmagnetic structure comprises a common mode core and a differential modecore with the common mode core comprising a higher permeability materialthan the differential core.
 14. The system of claim 13, wherein thecommon mode core comprises a closed rectangular core wound with threecommon mode coils, one for each phase, and wherein the differential modecore comprises an E core wound with a respective differential mode coilon each leg, the legs of the E core facing the closed rectangular coreand sharing a part of magnetic flux path of the closed rectangular core.15. The system of claim 13, wherein the common mode core comprises a topclosed rectangular core wound with three common mode top coils and abottom closed rectangular core wound with three common mode bottomcoils, and wherein the differential mode core comprises three posts witha respective differential mode coil on each post, the three postsarranged between the top and bottom closed rectangular cores and sharinga part of top and bottom rectangular magnetic flux paths.
 16. The systemof claim 12, wherein the respective phases of the three phase commonmode choke and three phase differential mode choke are connected inseries.
 17. The system of claim 11, wherein the integrated magneticchoke comprises an integrated magnetic structure coupling a single phasecommon mode choke and a single phase differential mode choke.
 18. Thesystem of claim 17, wherein the integrating magnetic structure comprisesa common mode core and a differential mode core with the common modecore comprising a higher permeability material than the differentialcore.
 19. The system of claim 18, wherein the common mode core comprisesa closed rectangular core wound with two common mode coils, and whereinthe differential mode core comprises a U core wound with twodifferential mode coils on each leg, the legs of the U core facing theclosed rectangular core and sharing a part of magnetic flux path of theclosed rectangular core.
 20. The system of claim 18, wherein the commonmode core comprises a top closed rectangular core wound with two commonmode top coils and a bottom closed rectangular core wound with twocommon mode bottom coils, and wherein the differential mode corecomprises two posts with a differential mode coil on each post, the twoposts arranged between the top and bottom closed rectangular cores andsharing a part of top and bottom rectangular magnetic flux paths. 21.The system of claim 17, wherein the single phase common mode choke andsingle phase differential mode choke are connected in series.
 22. Across current control system for multiple, parallel-coupled powerconverters, the cross current control system comprising: common modechokes, each coupled to a respective power converter and comprising anintegrated magnetic AC link choke; local cross current detectors, eachconfigured for obtaining a common mode cross current from a respectiveoutput line of a respective power converter; local cross currentfeedback controllers, each configured for receiving the common modecross currents from respective local cross current detectors,calculating a resultant cross current, and generating a local feedbackcontrol signal; a global feedforward controller, configured fordetecting switching patterns of the power converters and generatingcounter balance zero-sequence global feedforward control signals; andlocal converter controllers, each configured for using a respectivelocal feedback control signal and a respective global feedforwardcontrol signal to drive the respective power converter in accordancewith an interleaved switching pattern with respect to the other powerconverters.
 23. The system of claim 22, further comprising a globalcross current feedback controller, configured for receiving the commonmode cross currents from the local cross current detectors, calculatinga resulting global cross current, and generating global feedback controlsignals, wherein each of the local converter controllers is configuredfor using a respective global feedback control signal to drive therespective power converter.
 24. The system of claim 22, furthercomprising modulators, each configured for receiving a local convertercontroller signal from a respective local converter controller andgenerating a firing signal for driving a respective power converter. 25.The system of claim 22, wherein the integrated magnetic choke comprisesan integrating magnetic structure coupling a three phase common modechoke and a three phase differential mode choke.
 26. The system of claim25, wherein the integrating magnetic structure comprises a common modecore and a differential mode core with the common mode core comprising ahigher permeability material than the differential core.
 27. The systemof claim 26, wherein the common mode core comprises a closed rectangularcore wound with three common mode coils, one for each phase, and whereinthe differential mode core comprises an E core wound with a respectivedifferential mode coil on each leg, the legs of the E core facing theclosed rectangular core and sharing a part of magnetic flux path of theclosed rectangular core.
 28. The system of claim 26, wherein the commonmode core comprises a top closed rectangular core wound with threecommon mode top coils and a bottom closed rectangular core wound withthree common mode bottom coils, and wherein the differential mode corecomprises three posts with a respective differential mode coil on eachpost, the three posts arranged between the top and bottom closedrectangular cores and sharing a part of top and bottom rectangularmagnetic flux paths .
 29. The system of claim 22, wherein the integratedmagnetic choke comprises an integrated magnetic structure coupling asingle phase common mode choke and a single phase differential modechoke.
 30. The system of claim 29, wherein the integrating magneticstructure comprises a common mode core and a differential mode core, thecommon mode core comprising a higher permeability material than thedifferential core.
 31. A cross current control system for multiple,parallel-coupled power converters, the cross current control systemcomprising: common mode chokes, each coupled to a respective powerconverter; local cross current detectors, each configured for obtaininga common mode cross current from a respective output line of arespective power converter; local cross current feedback controllers,each configured for receiving the common mode cross currents fromrespective local cross current detectors, calculating a resultant crosscurrent, and generating a local feedback control signal; a globalfeedforward controller configured for detecting switching patterns ofthe power converters and generating counter balance zero-sequence globalfeedforward control signals; a global cross current feedback controllerconfigured for receiving the common mode cross currents from the localcross current detectors, calculating a resulting global cross current,and generating global feedback control signals; and local convertercontrollers, each is configured for using a respective local feedbackcontrol signal, a respective global feedback control signal and arespective global feedforward control signal, to drive the respectivepower converter in accordance with an interleaved switching pattern withrespect to the other power converters.
 32. A method of controllingcross-current through multiple, parallel-coupled power converters,comprising: providing common mode chokes, each coupled to a respectivepower converter; obtaining common mode cross currents from output linesof the power converters; and for each respective power converter,calculating a resultant cross current by using the respective commonmode cross currents, generating a local feedback control signal by usingthe resultant cross current, driving the respective power converter byusing the respective local feedback control signal in accordance with acoordinated switching pattern with respect to the other powerconverters.
 33. The method of claim 32, further comprising detectingswitching patterns of the power converters and generating counterbalance zero-sequence global feedforward control signals associated withrespective power converters, wherein driving the respective powerconverter comprises using the respective global feedforward controlsignal.
 34. The method of claim 33, further comprising obtaining commonmode cross currents from output lines of the power converters;calculating a resulting global cross current; and generating globalfeedback control signals, wherein driving the respective power convertercomprises using the respective global feedback control signal.
 35. Themethod of claim 32, further comprising obtaining common mode crosscurrents from output lines of the power converters; calculating aresulting global cross current, and generating global feedback controlsignals associated with respective power converters wherein driving therespective power converter comprises using the respective globalfeedback control signal.
 36. The method of claim 32, wherein providingcommon mode chokes comprises providing DC link chokes.
 37. The method ofclaim 32, wherein providing the common mode chokes comprises providingAC link chokes.
 38. The method of claim 37, wherein providing AC linkchokes comprises providing discrete magnetic chokes.
 39. The method ofclaim 38, wherein providing the AC link chokes comprises providingintegrated magnetic chokes.
 40. A method for controlling a powerconverter comprising: providing an integrated magnetic AC link commonmode choke coupled to the power converter; obtaining common modecurrents from respective output lines of the power converter; generatinga feedback control signal; and driving the power converter by using thefeedback control signal.